1. Field of the Invention
This invention relates to capped poly-2-pyrrolidone compositions having improved thermal stabilities and to methods of treating poly-2-pyrrolidone to prepare the capped poly-2 pyrrolidone compositions. In another aspect, this invention relates to methods of melt extruding, and especially melt spinning, such compositions.
2. The Prior Art
Poly-2-pyrrolidone is produced by the alkaline-catalyzed polymerization of poly-2-pyrrolidone in the presence of carbon dioxide (see U.S. Pat. No. 3,721,652). Polypyrrolidone so-produced can be melt-spun into filaments by extrusion from multihole spinnerets. In melt-spinning, the polymer composition is extruded in a molten condition at a melt temperature which is generally greater than about 270.degree. C. This extrusion must be carried out with care because of the propensity of the polymer to thermally degrade, reverting back to monomer. As well as causing substantial product loss and increasing process cost, this can also cause bubbles and the formation of voids or pox marks in the extrudate or filaments. Moreover, in addition to monomer reversion, molecular weight degradation also occurs during melt spinning resulting in poly-2-pyrrolidone filaments having substantially lower molecular weights than the original polymer. If the molecular weight of the initial polymer is too low or if the molecular weight degradation is too severe, the molecular weight of the filaments will be inadequate to afford the filaments sufficient tensile strength and fibrillation properties. (Mere molecular weight degradation in itself is not a significant problem unless the molecular weight is in fact degradated below the tensile strength-fibrillation limits.)
A further problem is that higher molecular weight (e.g., about 120,000 and above) polymers, although thermally more stable, are also more viscous and more difficult to extrude.
If extrusion is attempted at appreciably lower temperatures to avoid thermal decomposition, the material is not properly melted and fibers of substantially lower tensile strength are produced. Consequently, in order to melt extrude polypyrrolidone efficiently, one may either seek to increase the thermal stability of the polymer, or to improve the extrudability of the polymeric composition.
The unusual propensity of poly-2-pyrrolidone to thermally degrade back to its monomer and the adverse effect on melt spinning or melt extrusion is recognized to be a serious problem and the prior art has made a number of attempts to mitigate this problem. For example, U.S. Pat. No. 3,052,654 attempts to reduce degradation occurring during melt extrusion by pre-heating the poly-2-pyrrolidone to remove degradation products, e.g., pyrrolidone.
U.S. Pat. No. 3,026,301 teaches that the reaction of 2-pyrrolidone and hexamethylenediamine during polymerization produces a poly-2-pyrrolidone polymer having improved thermal stability but, that if the hexamethylenediamine is mixed with the poly-2-pyrrolidone after polymerization, no improvement is obtained.
U.S. Pat. No. 4,076,696 discloses that the addition of a small amount of certain alkylamines and alkyleneamines, including hexamethylenediamine, to poly-2-pyrrolidone permits the material to be extruded at a lower temperature and may be functioning as a slip agent.
U.S. Pat. No. 3,017,393 teaches that the melt extrusion degradation problem may be alleviated by washing or treating the polymer with an aqueous organic carboxylic acid solution and similarly, U.S. Pat. No. 3,072,615 teaches using dilute aqueous fatty acid.
U.S. Pat. No. 4,071,486 teaches that the thermal stability of poly-2-pyrrolidone can be improved by the addition of certain epoxides.
U.S. Pat. No. 3,009,893 teaches that the problem may be reduced by the incorporation of relatively large amounts of certain substances which apparently function as melting point depressants permitting nylon-4 to be extruded at lower temperatures.
U.S. Pat. No. 3,549,580 discloses a process for preparing spinnable solutions of poly-beta-amides by the direct polymerization of beta-lactams (e.g., azetinone) in a neutral or weakly basic solvent, containing 0.9 to 50% wt lithium halides or lithium pseudo halides, in the presence of catalysts and optional activators. Dimethyl formamide is disclosed as a preferred solvent for economic reasons.
Japanese Patent No. 43-27434 (1968) discloses a process for improving the thermal stability of polypyrrolidone which comprises treating polypyrrolidone with an alcohol in the presence of a strong inorganic acid or organic sulfonic acid and Japanese Patent No. 44-23509 (1969) discloses a two-step process wherein the polymer is first treated with alcohol, in the presence of a strong acid, and then treated with a monoisocyanate.
Japanese Patent No. 43-23637 (1968) discloses a process for improving the thermal stability of polypyrrolidone which comprises treating polypyrrolidone with an aqueous boric acid solution. Similarly, Japanese Patent No. 43-130059 (1968) discloses a two-step process wherein the product of the boric acid treatment is treated with an organic isocyanate.
Also, while not concerned with thermal stability, U.S. Pat. No. 3,042,659 discloses a process for polymerizing 2-pyrrolidone which comprises polymerizing 2-pyrrolidone in a mixture containing a catalytic amount of an alkaline polymerization catalyst, a few percent of an N-monocarbonyl-2-pyrrolidone, polymerization activator and various organic acid amides, including dimethyl formamide. This patent teaches that the N-monocarbonyl pyrrolidone when used in conjunction with certain organic amides, which are not themselves activators, are highly effective activators for the polymerization of 2-pyrrolidone and that the polymer product has a high molecular weight. The patent further teaches that the organic acid amides are not themselves activators but augment the activation activity of the N-monocarbonyl-2 pyrrolidone.